Allium cepa L.

Bulbs 1–3, not rhizomatous, mostly depressed-globose, varying in size from cultivar to cultivar, 5–8 × 3–10 cm; outer coats enclosing 1 or more bulbs, yellowish brown, red, or white, membranous, without reticulation; inner coats white to pink, cells obscure to quadrate. Leaves persistent, 4–10, sheathing proximal 1/6–1/4 scape; blade fistulose, usually ± semicircular in cross section, 10–50 cm × 4–20 mm. Scape persistent, solitary, erect, fistulose, inflated below middle, 30–100 cm × 3–20 mm. Umbel persistent, erect, compact, to 500-flowered, globose, bulbils occasionally found; spathe bracts caducous, 2–3, 3–4-veined, ovate, ± equal, apex acute to acuminate. Flowers stellate to campanulate to urceolate, 3–7 mm; tepals erect to ± spreading, white to pink with greenish midveins, withering in fruit, margins entire, apex obtuse or acute, outer ovate, inner oblong; stamens exserted; anthers white; pollen white; ovary crestless; style linear, ± equaling stamens; stigma capitate, unlobed; pedicel 10–50 mm. Seed coat not known.

A temperate plant. It is more easy to get onions to grow and form bulbs in the highlands in the tropics but they can be grown from sea level to 2600 m altitude in Papua New Guinea. In Nepal they grow to 3000 m altitude. They do not suit the wet humid tropics. Because the day lengths are short near the equator, onions often do not produce a bulb. Some newer varieties will yield in the tropics. It plant grows best at 12-21°C and the bulb forms best at 15-25°C. It grows best with a pH 5.7-6.6. It suits hardiness zones 5-10.

The bulbs and leaves are used as flavouring raw or cooked. The flowers are crushed and eaten.

They are grown from imported seed. For bulbs, a tropical cultivar is needed and bulbing is normally better at higher altitudes. Cultivars which form flowers early need to be avoided. Seedlings can be transplanted.